The present invention generally relates to alternative energy systems, and particularly relates to the control of such systems.
Alternative energy systems represent an increasingly viable alternative to or supplement to conventional utility grid power. For example, advancements in power generation and control technologies have increased the power density and reliability of fuel cell based generation systems such that those types of systems are increasingly realistic for use in providing localized power for residences and small business, and even in providing opportunities for cogeneration wherein a grid-connected alternative energy system injects a portion of its output power into the utility grid.
Typically, such systems convert dc power from a fuel cell (or other local source) into ac power using a power inverter. Further, such systems typically include one or more batteries to provide backup power, and, at least on a temporary basis, to provide higher power than can be provided by the primary source, e.g., fuel cell. As such, the typical alternative energy system includes multiple energy sources (e.g. fuel cell and backup batteries) and it is necessary to be able to regulate more than one power system variable. Examples of variables of interest include, but are not limited to, fuel cell current, battery current and/or voltage, and dc bus voltage.
Controlling such systems based on a plurality of different, potentially conflicting, system variables presents a significant control challenge, particularly where the systems operate in and switch between a variety of modes, such as stand alone, grid-connected, etc. For example, in certain operating modes, there can be one or a limited number of control elements within the system that can be used to effect the desired regulation.